It is known that the aftertreatment system of a turbocharged internal combustion engine can be provided, among other devices, with a Lean NOx Trap (LNT) which represents a cost effective alternative to Selective Catalytic Reduction (SCR). The LNT is a catalytic device containing catalysts, such as Rhodium, Platinum and/or Palladium, and adsorbents, such as barium based elements, which provide active sites suitable for binding and trapping the nitrogen oxides (NOx) contained in the exhaust gas. In order to periodically restore its original efficiency, the Lean NOx Trap (LNT) needs to be subjected to a regeneration process or event, whereby the trapped nitrogen oxides (NOx) are reduced and desorbed. This regeneration event is performed by switching the internal combustion engine from a lean operation mode to a rich operation mode (e.g. lambda value of 0.95 or close to 1), so that the NOx stored on the adsorbent active sites of the LNT are desorbed and reduced by the reductants contained in the exhaust gas.
For the purpose of guaranteeing safe regeneration events, the electronic control unit of the internal combustion engine is generally configured to activate the regeneration event only if the operating point of the internal combustion engine is confined into a pre-calibrated area, i.e. a so-called rich mode area, of the engine speed—engine torque diagram. In particular, the rich mode area is delimited by an upper curve, in order to guarantee that, over predetermined upper threshold values of the engine torque (for example 230-250 Nm), the regeneration events are stopped or avoided. In fact, a regeneration event performed while the engine is operating above the upper curve could subject the aftertreatment devices to high thermal stresses, particularly because the current control strategies of the turbocharger do not permit to control the boost pressure with the necessary precision. However, it has been observed that a regeneration event performed at high load conditions may be more efficient in terms of NOx conversion and may result in lower fuel consumption, so that an upper extension of the rich mode area is generally desirable.